The present invention relates to an approximate third-order function generator, for example, for use with a temperature compensated crystal oscillator, a temperature compensated crystal oscillation circuit made by using the function generator, and a temperature compensation method.
An example of an approximate third-order function generator of this type is the invention described, for example, on Japanese Patent Laid-Open No. 9-55624 which the present applicant proposed before.
This prior art discloses an approximate third-order curve generation circuit which comprises three differential amplifiers, each comprising a pair of MOS field effect transistors receiving a common first-order input signal and a fixed level signal, the respective fixed level signals to the three differential amplifiers being different in level, and providing a non-inverted output and an inverted output, and the non-inverted outputs and the inverted outputs from the three differential amplifiers being added respectively in group.
However, although the above prior art generator can generate an approximate third-order function, it cannot arbitrarily control its variable independently, which is a unsolved problem.
A general form of the third-order function is represented by:
f(x)=a3x3+a2x2+a1x+a0xe2x80x83xe2x80x83(1)
By transforming the variable in the expression (1), the expression (1) is rewritten as follows:
f(x)=a3xe2x80x2(xxe2x88x92x0)3+a1xe2x80x2(xxe2x88x92x0)+a0xe2x80x2
a3=a3xe2x80x2
a23xe2x80x2x0
a1=3a3xe2x80x2x02+a1xe2x80x2
a0=a0xe2x80x2xe2x88x92a3x03xe2x88x92a1xe2x80x2x0xe2x80x83xe2x80x83(2)
Since in the prior art the three differential amplifiers are used to generate an approximate third-order function, the output approximate third-order function is shown in FIG. 15, which is represented by the following expression (3).
This expression (3) contains a second term component of the expression (2) thereby and variables a3xe2x80x2 and a1xe2x80x2 in the expression (2) cannot be controlled independently, which is an unsolved problem.
f(x)=xcex1{a3xe2x80x3(xxe2x88x92x0)3+a1xe2x80x3(xxe2x88x92x0)}+axe2x80x3xe2x80x83xe2x80x83(3)
Especially, when the approximate third-order function generator is used for temperature compensation of an voltage-controlled crystal oscillation circuit, an approximate third-order function which compensates a temperature characteristics of the crystal oscillator is required to be generated, but an accurate temperature compensated crystal oscillation circuit cannot be constituted, which is an unsolved problem.
The present invention is made in view of those unsolved problems with the prior art. It is a first object of the present invention to provide an approximate third-order function generator capable of accurately outputting only a first term component of the expression (2) and controlling the respective variables independently.
A second object of the present invention is to provide a temperature compensated crystal oscillation circuit which uses an approximate third-order function generator to perform accurate temperature compensation.
A third object of the present invention. is to provide a method of performing temperature compensation for a temperature compensated crystal oscillation circuit.
In order to achieve the first object, an approximate third-order function generator of claim 1 comprises a first amplifier, a second amplifier and a third amplifier each receiving a common input signal and a different fixed level signal, the three different fixed level signals received respectively by the first, second and third amplifiers sequentially increasing in level in this order, each amplifier having an input-output characteristics in which a non-inverted or inverted output signal is provided based on the common input signal and the fixed level signal concerned, that amplifier also having a function to limit the output signal to within a range defined by a maximum predetermined value and a minimum predetermined value; a fourth amplifier receiving the common input signal and the same fixed level signal as the second amplifier receives, the fourth amplifier having an input-output characteristics in which a non-inverted or inverted output signal is provided based on the common input signal and the fixed level signal concerned, and having a function to limit the output signal to within a range defined by a maximum predetermined value and a minimum predetermined value; and a fixed level signal generation circuit for supplying the fixed level signals having the different fixed levels to the first, second, third, and fourth amplifiers, respectively, the output characteristics of the first, third and fourth amplifiers being set so as to have the same polarity, the output characteristics of the second amplifier being set so as to be reverse to those of the first, third and fourth amplifiers, wherein the output signals from the first, second, third and fourth amplifiers being added to generate a third-order function component free from a first-order component.
In the invention of claim 1, the second amplifier which has a characteristics reverse to those of the first, third and fourth amplifiers is provided, so that only a third-order component free from the first-order component as the first term of the expression (2) produced by the first, second and third amplifiers can be generated. Thus, the variables of the first and second terms of the expression (2) can be controlled independently.
An approximate third-order function generator of claim 2 comprises a third-order component generation unit which includes a third-order component generation circuit having the composition of claim 1 supplied with an input added voltage as a first-order input voltage which includes the sum of a first-order input voltage signal and a variable voltage signal, and a variable gain amplifier which receives an amplified version of the differential between the non-inverted and inverted output signals from the third-order component generation circuit; a first-order component generation unit for receiving the input added voltage and for generating a first-order component; a constant generation unit for receiving a constant voltage signal and for generating a constant component; and an addition circuit for adding output signals from the third-order component generation unit, the first-order generation unit, and the constant generation unit.
The invention of the claim 2 is constructed so as to comprise the third-order component generator, the first-order component generation unit and the constant generation unit which have the respective compositions described in claim 1 and their outputs are added. Thus, the third-order function of the expression (2) can be generated accurately and the respective variables can be controlled independently.
An approximate third-order function generator of claim 3 is characterized in that in the invention of claim 1 or 2, the fourth amplifier has an output characteristics reverse in inclination to the inverted output characteristics of the second amplifier, and the distance between the maximum and minimum values of the output signal is set so as to be longer than that of the second amplifier.
In the invention of claim 3, an approximate first-order straight line can be generated in a range of input voltages which can be approximated by a third-order function which has been generated by the first-third amplifiers to thereby ensure that the first-order component contained in the third-order component is offset.
An approximate third-order function generator of claim 4 is characterized in that the first-fourth amplifiers each comprise a differential amplifier having a pair of MOS field effect transistors.
In the invention of claim 4, the approximate third-order function generator includes CMOSs to thereby achieve higher density integration and reduced power consumption.
A temperature compensated crystal oscillation circuit of claim 5 is characterized by a temperature detection circuit, a temperature compensation circuit which includes an approximate third-order function generator according to any one of claims 1-4 for receiving a detection signal from the temperature detection circuit, and a voltage-controlled crystal oscillation circuit for receiving the approximate third-order function generated by the temperature compensation circuit.
In the invention of claim 5, an accurate third-order function which is free from first-order component contained in the third-order component is generated, as described above, in the approximate third-order function generator of the temperature compensation circuit. The temperature characteristics of the crystal oscillator in the voltage-controlled crystal oscillation circuit is compensated accurately.
A temperature compensation adjusting method for a temperature compensated crystal oscillation circuit of claim 6 comprises the steps of measuring an output voltage VCout from a temperature compensation circuit in a predetermined temperature atmosphere; measuring an input voltage VCin to the voltage-controlled crystal oscillation circuit where the oscillation frequency output from the voltage-controlled crystal oscillation circuit coincides with a preset selected frequency at a respective one of a plurality of temperatures T in a desired temperature compensation range; approximately representing the measured input voltage VCin and output voltage VCout at the respective temperature by:
VCin(T)=xcex13(Txe2x88x92T0)3+xcex11(Txe2x88x92T0)+xcex10
VCout(T)=xcex23(Txe2x88x92T0xe2x80x2)3+xcex21(Txe2x88x92T0xe2x80x2)+xcex20;
and adjusting coefficients xcex20, xcex21, xcex23 and T0xe2x80x2 of the temperature compensation circuit so as to coincide with the coefficients xcex10, xcex11, xcex13 and T0, respectively, inherent to a crystal resonator of the voltage-controlled crystal oscillation circuit.
In the invention of claim 6, the output voltage VCout from the temperature compensation circuit and the input voltage VCin to the voltage-controlled crystal oscillation circuit are measured at the respective one of the plurality of temperatures in the desired temperature compensation range, and approximated by the corresponding third-order function expressions each as a function of a temperature. The coefficients of the temperature compensation circuit are adjusted so as to coincide with coefficients dependent on the crystal resonator of the voltage-controlled oscillation circuit and hence the temperature compensation is achieved by a single temperature sweeping operation.
As described above, according to the invention of claim 1, the approximate third-order function generator comprises a first amplifier, a second amplifier and a third amplifier, each receiving a common input signal and a different fixed level signal, the three different fixed level signals received by the first, second and third amplifiers sequentially increasing in level in this order, each amplifier having an input-output characteristics in which a non-inverted or inverted output signal is provided based on the common input signal and the fixed level signal concerned, that amplifier also having a function to limit the output signal to within a range defined by a maximum predetermined value and a minimum predetermined value; a fourth amplifier receiving the common input signal and the same fixed level signal as the second amplifier receives, the fourth amplifier having an input-output characteristics in which a non-inverted or inverted output signal is provided based on the common input signal and the fixed level signal concerned, and having a function to limit the output signal to within a range defined by an maximum predetermined value and a minimum predetermined value; and a fixed level signal generation circuit for supplying the fixed level signals having the different fixed levels to the first-fourth amplifiers, respectively, the output characteristics of the first, third and fourth amplifiers being set so as to have the same polarity, the output characteristics of the second amplifier being set so as to be reverse to those of the first, third and fourth amplifiers, the output signals from the first, second, third and fourth amplifiers being added to generate a third-order function component free from a first-order component. Thus, only the third-order component free from the first-order component in the third-order function is output. Thus, the variables of the third-order function can be controlled independently.
According to the invention of claim 2, the approximate third-order function generator comprises a third-order component generation unit which includes a third-order component generation circuit having the composition of claim 1 supplied with an input added voltage as a first-order input voltage which includes the sum of a first-order input voltage signal and a variable voltage signal, and a variable gain amplifier which receives an amplified version of the differential between the non-inverted and inverted output signals from the third-order component generation circuit; a first-order component generation unit for receiving the input added voltage and for generating a first-order component; a constant generation unit for receiving a constant voltage signal and for generating a constant component; and an addition circuit for adding output signals from the third-order component generation unit, the first-order generation unit, and the constant generation unit. Thus, by controlling independently the respective variables of a third-order component, a first-order component and a constant component obtained when a general-form third-order function is converted with respect to its variables, any third-order function is realized as a voltage input and a voltage output.
According to the invention of claim 3, the fourth amplifier has an output characteristics reverse in inclination to the inverted output characteristics of the second amplifier, and the distance between the maximum and minimum values of the output signal is set so as to be longer than that of the second amplifier. Thus, an approximate first-order straight line can be generated in a range of input voltages which can be approximated by a third-order function which has been generated by the first-third amplifiers to thereby ensure that the first-order component contained in the third-order component is offset.
According to the invention of claim 4, the first-fourth amplifiers each comprise a differential amplifier having a pair of MOS field effect transistors. Thus, the whole approximate third-order function generator has a CMOS composition to thereby achieve higher density integration and reduced power consumption.
According to the invention of claim 5, the temperature compensated crystal oscillation circuit comprises a temperature detection circuit, a temperature compensation circuit which includes an approximate third-order function generator according to any one of claims 1-4 for receiving a detection signal from the temperature detection circuit, and a voltage-controlled crystal oscillation circuit for receiving the approximate third-order function generated by the temperature compensation circuit. Thus, an accurate third-order function which is free from first-order component contained in the third-order component is generated, as described above, in the approximate third-order function generator of the temperature compensation circuit. Thus, the temperature characteristics of the crystal oscillator in the voltage-controlled crystal oscillation circuit is compensated accurately.
According to the invention of claim 6, the output voltage VCout from the temperature compensation circuit and the input voltage VCin to the voltage-controlled crystal oscillation circuit are measured at the respective one of the plurality of temperatures in the desired temperature compensation range, and approximated by the corresponding third-order function expressions each as a function of a temperature. The coefficients of the temperature compensation circuit are then adjusted so as to coincide with coefficients dependent on the crystal resonator of the voltage-controlled crystal circuit and hence a high-accuracy temperature compensation is achieved by a single temperature sweeping operation.